1. Field of the Invention
The present invention relates to driving circuits of an inverter, and more particularly, to prevention of malfunctions of driving circuits of low-side switching devices.
2. Description of the Background Art
FIG. 6 is a circuit diagram showing a conventional single-phase inverter. Here, driving circuits and protection circuits of low-side switching devices 101A and 101B are illustrated, whereas those of high-side switching devices 102A and 102B are not illustrated.
The low-side switching devices 101A, 101B and high-side switching devices 102A, 102B are IGBTs (insulated gate bipolar transistors), to which diodes 103A, 103B, 104A and 104B are connected in parallel, respectively.
The emitter terminals of the low-side switching devices 101A and 101B are both grounded through an N bus (N), while the source terminals of the high-side switching devices 102A and 102B are both connected to a power source 105 through a P bus (P). The source terminals of the low-side switching devices 101A and 101B and the emitter terminals of the high-side switching devices 102A and 102B are all connected to a load.
Next, the low-side switching devices 101A and 101B are each connected to a driving circuit and a protection circuit. The driving circuits are formed by input buffers 106A, 106B and emitter follower circuits 107A, 107B, 108A and 108B. Control signals to the respective switching devices 101A and 101B are amplified in voltage at the input buffers 106A and 106B, respectively, and are supplemented in current driving capability at the emitter follower circuits 107A, 108A and at 107B, 108B, respectively, to be inputted to the base terminals of the low-side switching devices 101A and 101B, respectively. Resistors 109A and 109B are provided between the emitter follower circuit 108A and the base terminal of the low-side switching device 101A, and between the emitter follower circuit 108B and the base terminal of the low-side switching device 101B, respectively, while resistors 110A and 110B are provided between the emitter follower circuit 107A and the base terminal of the low-side switching device 101A, and between the emitter follower circuit 107B and the base terminal of the low-side switching device 101B, respectively.
The protection circuits are formed by input buffers 111A, 111B and resistors. The driving circuits and protection circuits connected to these low-side switching devices 101A and 101B are all connected to a common power source 112. A ground line of the driving circuits and protection circuits is connected to the emitter terminals of the low-side switching devices 101A and 101B. Thus, the N bus (N) and ground line of the driving circuits and protection circuits form an electric loop through the emitter terminals of the low-side switching devices 101A and 101B. Further, a power line of the driving circuits and protection circuits is connected to the ground line through power by-pass capacitors 113A, 113B, 114A and 114B.
The emitter terminals of the low-side switching devices 101A and 101B are each connected to the N bus (N). On this N bus (N), self-inductances 115A and 115B are present. Thus, switching of the low-side switching devices 101A and 101B changes the amount of current running through the N bus (N), causing a surge voltage to occur. The surge voltage changes the potential of the ground line of the driving circuits and protection circuits which form the electric loop, and also changes the voltage of the power line of the driving circuits and protection circuits connected through the power by-pass capacitors 113A, 113B, 114A and 114B. This causes the driving circuits and protection circuits to malfunction.
In the case where the driving circuits and protection circuits connected to the switching devices of the inverter are connected to the common power source 112 and the ground line of the driving circuits and protection circuits and the N bus (N) form an electric loop as described above, the occurrence of a surge voltage due to the self-inductances present on the N bus (N) causes variations in the voltage of the ground line and power line of the driving circuits and protection circuits, causing the driving circuits and protection circuits to malfunction.
Conventionally, there has been a method of providing a power source for each of driving circuits and protection circuits of low-side switching devices in order to solve the aforementioned drawbacks. FIG. 7 is a circuit diagram showing a single-phase inverter in which driving circuits and protection circuits are each provided with a power source. The low-side switching devices 101A, 101B, high-side switching devices 102A, 102B, driving circuits and protection circuits shown in FIG. 7 have the same configuration as those shown in FIG. 6.
The driving circuit and protection circuit of the low-side switching device 101A are connected to a power source 116A, while those of the low-side switching device 101B are connected to a power source 116B. This is the difference from the configuration shown in FIG. 6. With the configuration shown in FIG. 7, the ground line of the driving circuits and protection circuits and N bus (N) do not form an electric loop. Thus, a surge voltage as generated does not change the voltages of the ground line and power line of the driving circuits and protection circuits, preventing the driving circuits and protection circuits from malfunctioning.
With the configuration shown in FIG. 7, however, the low-side switching devices and high-side switching devices each require a power source. For instance, a three-phase inverter requires six low-side switching devices and high-side switching devices in total, thus requiring six power sources in total. Therefore, drawbacks arise such as cost increase due to provision of additional power sources, upsizing of power sources and increase in interconnection between power sources, driving circuits and protection circuits.
An object of the present invention is to provide an inverter driving circuit capable of preventing a malfunction due to a surge voltage without upsizing circuits or increasing costs.
According to the present invention, a power device driving circuit includes a plurality of low-side switching devices, a plurality of driving circuits, a power source, an interconnect wire and a first resistor. The plurality of low-side switching devices form an inverter circuit, each having one terminal connected to a load and the other terminal connected to a common low-potential line. The plurality of driving circuits are configured to drive the plurality of low-side switching devices, respectively. The plurality of driving circuits are commonly connected to the power source. The interconnect wire is configured to connect the plurality of driving circuits and the power source, and to form an electric loop with the low-potential line through the plurality of low-side switching devices. The first resistor is provided on the interconnect wire between the plurality of driving circuits and the power source.
Since the power device driving circuit includes the resistor on the interconnect wire between the plurality of driving circuits and the power source, a surge current is unlikely to run through the interconnect wire, which can prevent the driving circuits and protection circuits from malfunctioning.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.